Two precocious American teenage boys call the Pope for a prank. One says he is Henry Kissinger.

It's the middle of the night in the Vatican but the Pope's representatives scurry off to wake him. Only when the boys start laughing do they realise it is a hoax.

The boys are Steve Jobs and his friend Steve Wozniak. They are using a "blue box", a device that enables them to imitate tones used for long-distance calls and to ring around the world for free.

The Apple founder, who died last October, tells this story with glee in Steve Jobs – The Lost Interview, a documentary premiering in Cannes next month. The film is based around an interview he gave to technology journalist Bob Cringely in 1995 for a Channel 4 series, Triumph Of The Nerds.

Only a few minutes were used. Jobs was one of 125 people Cringely spoke to. But once the series was broadcast, the master tapes went missing.

Last year, after Jobs' death, the series director, British filmmaker Paul Sen, went to his garage to look for his VHS version of the interview. He had kept it for 16 years, through several moves of house and despite his wife urging him to throw out his old junk.

"I remember being sat in the room at the time and thinking, 'This guy's an amazing interviewee,' " Sen says. "He had this amazing charisma. As soon as he walked in the room, we all noticed.

"We interviewed Bill Gates at the same time and he slipped into the room without you knowing he was there."

Cringely says the interview – most of which has not been seen before – is perhaps the most comprehensive and revealing Jobs ever gave.

At the time he was at a low ebb, working at NeXT, the company he founded after he left Apple following boardroom battles. Jobs reminisces about encountering his first computer aged 10 or 11.

"They were these big boxes...they were very mysterious, powerful things that did something in the background."

He recalls how at 12 he rang up Bill Hewlett of Hewlett-Packard and told him he was building a frequency counter and needed spare parts. (Hewlett provided them).

Jobs also strikes a rare self-deprecatory note, recalling how one venture capitalist he asked for backing called him "a renegade from the human race".

Evident throughout is his proselytising enthusiasm. He says: "I think everybody in this country should learn how to programme a computer because it teaches you how to think. Computer science should be a liberal art."

Cringely was "hired and fired three times" by Jobs at Apple in the late 70s and early 80s. He calls his former boss "a narcissist and an asshole" but adds: "He was a very effective leader and he changed the world."

Jobs also talks about the shortcomings of rivals such as Microsoft ("absolutely no taste"); lays out how he deals with people, products and innovation; and tells how he draws a line between creativity and marketing.

Cringely says: "It's a masterclass. This thing is going to be running in business schools for the next century."